A great number of current electrical appliances operate on direct current, and thus need alternating-direct current conversion since public electricity is alternating current. To reduce reactive power of an electronic system as well as to minimize current harmonics that cause system interference, a power factor corrector is prevailingly implemented in many electrical appliances that are required to have a high power factor and low current harmonics. A common power factor correcting circuit stereotypically adopts a boost approach, which is however set back by a limitation that a direct-current output voltage is necessarily higher than a peak value of an alternating-current input voltage. Further, although other circuits capable of outputting a lower voltage by means of buck or boost are available, these circuits suffer from drawbacks from having less satisfactory characteristics and efficiency, a large volume for a corresponding storage component, complex control means to low feasibilities.
FIG. 1A shows a boost converter circuit frequently adopted by a conventional power factor corrector, which is advantaged by having a higher power factor and simpler control means. FIG. 1B shows a schematic diagram of waveforms of an input voltage Vs and a current Is of the conventional power factor corrector in FIG. 1A, where ω is an angular frequency of public electricity, and Vm and Im respectively represent a voltage peak and a current peak. In an optimal situation, the power factor of the input current Is may approach 1.0. However, in the actual circuit application, the actual value of the power factor of the input current may approach above 0.98. In respect to the control of the switch element QPFC, the current in the store energy inductor LPFC is the input current Is and the charged voltage cross the strong energy inductor LPFC is equal to the AC source under the boundary current mode (BCM) (as shown in the equation (1)). After filtering the component of the high frequency of the input current Is the input current Is is directly proportional to the input voltage Vs (as shown in the equation (2)), so as to accomplish the high power factor and perform the control of the boost power factor corrector.
                              i                      s            ⁡                          (              peak              )                                      =                              i                                          L                PFC                            ⁡                              (                peak                )                                              =                                                                      2                                ⁢                                  v                  s                                ⁢                sin                ⁢                                                                  ⁢                θ                                            L                PFC                                      ⁢                          DT              s                                                          (        1        )                                          i                      s            ⁡                          (              avg              )                                      =                              i                                          L                PFC                            ⁡                              (                avg                )                                              =                                                                      2                                ⁢                                  v                  s                                ⁢                sin                ⁢                                                                  ⁢                θ                                            2                ⁢                                  L                  PFC                                                      ⁢                          DT              s                                                          (        2        )            
The two common methods to raise the power factor of the boost converter circuit: 1. zero-current cut and the constant on-time; 2. zero-current cut and double packets current cut-off. The two common methods both can implement the needed frequency and duty-cycle of the equation (2). Meanwhile, there are many ready-made IC accomplishing the two common methods. FIG. 1C illustrates relationship diagram among the input current Is and the inductor current IL of the boost converter circuit.
However, the limitation of the boost power factor corrector (or boost converter circuit) is that the input voltage must higher than the input voltage (peak voltage), and accompanied with a high output voltage, power components of the above conventional power factor corrector are often encountered with a higher voltage stress. In addition, for a load with a lower voltage requirement (lower than a peak voltage of the power source), the conventional boost power factor corrector, instead of directly providing an appropriate power source, is only able to provide a rated voltage needed by the load after stepping down its output voltage via a buck converting circuit, as shown in FIG. 2. Yet, the above design increases a circuit size and production costs as well as circuit power consumption, such that conversion efficiency of an overall circuit is reduced as a result.
To optimize conversion efficiency of a circuit, a power factor corrector with a design of a buck converter circuit has also been proposed, as shown in FIG. 3A. The major disadvantage of the buck power factor correction device is: the circuit can not receive the input current when the input voltage Vi from the AC source is lower than the output voltage Vo, and this phenomena is called “dead zone”, as shown in FIG. 3B. Thereby, the input current Is of the buck power factor correction device is discontinuous current, resulting in it has lower power factor and higher current resonance. Moreover, the bigger dead zone to cause the higher distortion rate and the current resonance, as shown in FIG. 3C.
Another disadvantage of the buck power factor correction device is that of complicated control manner of the switch element being difficult to achieve. This is because the inductor current of the buck power factor correction device only flows through the AC source at the charged section, as shown in the equation (3). After filtering the component of the high frequency of the input current Is the input current Is is not directly proportional to the input voltage Vs (as shown in the equation (4)). At present, there is no the ready-made ICs or the control circuits accomplishing and overcoming the advantages of the buck power factor correction device.
                              i                      s            ⁡                          (              peak              )                                      =                              i                                          L                PFC                            ⁡                              (                peak                )                                              =                                                    (                                                                            2                                        ⁢                                          v                      s                                        ⁢                    sin                    ⁢                                                                                  ⁢                    θ                                    -                                      V                    o                                                  )                                            L                PFC                                      ⁢                          DT              s                                                          (        3        )                                          i                      s            ⁡                          (              avg              )                                      =                              i                                          L                PFC                            ⁡                              (                avg                )                                              =                                                    (                                                                            2                                        ⁢                                          v                      s                                        ⁢                    sin                    ⁢                                                                                  ⁢                    θ                                    -                                      V                    o                                                  )                                            2                ⁢                                  L                  PFC                                                      ⁢                          D              2                        ⁢                          T              s                                                          (        4        )            
There is also a buck-boost converter circuit (as shown in FIG. 4) or a fly-back converter circuit (as shown in FIG. 5) for serving as a power factor corrector. The two types of converting circuits above although indeed achieve a large power factor, due to the fact that the current path in converting circuits does not allow energy from the power source to directly charge the direct-current link capacitor, they are both disadvantaged by having a larger storage requirement for the inductor, a larger volume and poorer efficiency caused by magnetic energy loss.
There is yet another power factor corrector formed by integrating a boost converter circuit and a buck converter circuit, as shown in FIG. 6. An active switch transistor Q1 performs buck conversion when an active switch transistor Q2 is off; the active switch transistor Q2 performs boost conversion when the active switch transistor Q1 is on. However, unless being implemented in a customized integrated for a specific use, such design is extreme complex and is rather highly unfeasible and unpractical.